1. Field of the Invention
This invention relates to circuit breakers having a trip bar which unlatches a latchable operating mechanism to open separable contacts in the circuit breaker. More particularly, it relates to arrangements for using arc gases generated during interruption to speed opening of the circuit breaker and/or for protecting moving parts from debris in the arc gases.
2. Background of the Invention
A common type of multi-phase circuit breaker has a molded case with side-by-side compartments for each of the poles. Each pole has separable contacts including a fixed contact and a moveable contact. The moveable contact is carried by a moveable contact assembly. The moveable contact assemblies of all of the poles are joined by crossbar for simultaneous opening and closing of the contacts in all of the poles. A single operating mechanism is coupled to the crossbar for manual opening and closing of the contacts through manipulation of a circuit breaker handle. The contacts can also be tripped open automatically by a trip unit which monitors the current in each of the poles. The trip unit includes a rotatably mounted trip bar which latches the operating mechanism. Upon detection of an overcurrent condition, a trip unit rotates the trip bar to unlatch the operating mechanism which then opens the contacts and all of the poles through rotation of the crossbar.
A certain amount of time is required for the trip unit to respond to the overcurrent condition and for the operating mechanism to initiate opening of the contacts after being unlatched by the trip bar. In order to speed up tripping in response to short circuits or other very high overcurrents, it is common to provide the moveable contact assemblies with a blow-open feature. Typically, the moving contact assembly includes a base member which is coupled to or formed integrally with the crossbar and therefore rotated by the crossbar. The moveable contact is affixed to the free end of a contact arm which is pivotally connected to the base member by a blow-open coupling. With the contacts closed, the contact arm is arranged in close proximity to a portion of the line conductor to which the fixed contact is attached and in which the current flows in a direction opposite to the direction through the contact arm. The magnetic fields produced by the oppositely flowing currents generate a very large repulsion force which rotates the contact arm relative to the base member and therefore blows the contacts open. The trip unit and operating mechanism subsequently respond and recouple the contact arm to the base member. This blow-open feature has been very effective in responding to very high overcurrents. However, there is room for improvement.
The arcs generated during interruption of a short circuit generate gases which typically contain debris in the form of vaporized contact material. This debris can become deposited on other components of the circuit breaker and have a detrimental effect on the operation of mechanical parts. In particular, debris deposited from arc gases on an electromechanical trip device can increase friction and result in poor performance. Hence, there is also room for improvement in the control of arc gases in circuit breakers.